This invention relates to keypads, and more particularly to instrument keypads that detect applied pressure by electrical contact between two membrane circuits that are normally spaced slightly apart.
FIG. 1 shows a prior art construction of a keypad based on two membrane circuits that are normally spaced slightly apart. The keypad is shown electrically connected to a circuit board 28. For clarity, the layers in this cross-sectional view have been shown spaced further apart than they actually are in an actual assembly.
An outer cover 10 of elastomeric material provides a suitable key appearance and tactile feedback when a finger applies pressure to one of the raised regions 12. Depressing the raised region 12 causes a thick spot 14 on the opposite surface of the cover 10 to contact a first membrane 16. A row of shorting conductors 18 on the bottom surface of the first membrane 16 is normally separated from transversely oriented rows of main conductors 22, 23 on a second membrane 24 by a third membrane 20. The third membrane 20 has cut-away areas in the vicinity of the rows of shorting conductors 18 that allow those rows of shorting conductors to make contact with the transversely oriented rows of main conductors 22, 23 on the second membrane 24 when the raised region 12 of the cover 10 is depressed. When the raised region 12 is not depressed, the thickness of the third membrane 20 and the stiffness of the first and second membranes 16 and 24 prevent contact between the shorting conductors 18 and the main conductors 22 and 23. A stiff backing element 26 provides a surface for the membrane layers 16, 20, 24 and their conductors 18, 22, 23 to be compressed against.
Referring now to FIG. 2, the rows of shorting conductors 18 on the first membrane are shown in their transverse relationship to rows of main conductors 22 and 23 on the second membrane. In this view, it can be seen that when the rows of shorting conductors 18 on the first membrane are pressed downward, they make contact with both rows of main conductors 22 and 23 on the second membrane, shorting them together. This electrical contact between the first set of main conductors 22 and the second set of main conductors 23 is detected by circuitry connected to extensions 22' and 23' of the main conductors 22 and 23.
Returning now to FIG. 1, the contact between the rows of conductors 18, 22 and 23 described above in connection with FIG. 2 must be communicated to circuitry (represented by conductors 30, 31) on a printed wiring board 28 so that other circuitry located there (not shown) can respond to key activity. Extensions 24' and 20' of the second membrane 24 and the third membrane 20, shield extensions 22' and 23' of the conductors 22 and 23 in what is known as a "tail". This "tail" must pass through a slot 25 in the stiff backing element 26, 26' and terminate in a male connector 32. This male connector 32 then must be mated with a female receptacle 33 on the printed wiring board 28, thereby bringing the main conductors 22 and 23 of the membrane assembly into contact with conductors 30 and 31 on the printed wiring board 28 via the extension conductors 22',23'.
While this prior art approach works, the need to fit the "tail" of the membrane assembly with the male connector 32, pass this male connector 32 and the "tail" of the membrane assembly through the slot 25 in the stiff backing member 26, 26', and then bend it down and around and into contact with the female receptacle 33 on the printed wiring board 28 creates undesirable manufacturing complexity.
In product designs optimized for manufacturability, especially highly automated methods of manufacture, "Z-axis assembly" principles are proving to be very important. Z-axis assembly simply means that a product is assembled by lowering the parts from above onto an existing sub-assembly. This is especially important in robotic assembly, but many of the same benefits can also be realized even in manual assembly. If automated assembly is being employed, the simplest and most cost effective parts handlers and other robotic machines can be employed to stack and connect parts to an existing sub-assembly quickly and easily if the product has been designed for Z-axis assembly.
With the foregoing in mind, the problem that arises in connection with the manufacturing of the prior art keypad shown in FIG. 1 can be better appreciated. The need to fit the extensions 24' and 20' of the membrane and the conductors 22' and 23' associated with them into the male connector 32, and then fit that male connector 32 through the slot 25 and down and around into contact with the female receptacle 33, violates the principle of Z-axis assembly and necessitates a human role in the product's manufacture.
What is desired is a structure and method for making keypads that eliminates the need for a membrane assembly "tail" or other jumper-like means of connection between the keypad and other circuitry in the instrument.